1. Field of Invention
The invention relates to a shift control apparatus for an automatic transmission. In particular, the invention relates to a technique for smoothly performing a shift requiring disengagement and engagement of two different engagement elements between shift stages (simultaneous grip change of four elements).
2. Description of Related Art
In an automatic transmission, it has been well known that a power transmission route via a shift element composed of planetary gears is switched by engaging and disengaging friction engagement elements, thereby achieving a plurality of shift stages by changing a gear ratio. Engagement and disengagement of the engagement elements during shifting must be performed under a hydraulic control that is as simple as possible while reducing shock resulting from shifting. Thus, in general, operation of engagement elements for up-shift and down-shift basically includes additional engagement of another engagement element or disengagement of at least one engagement element being engaged. If the operation cannot be performed because of the gear train construction, one engagement element that has been already engaged is disengaged so as to engage another engagement element, that is, a so called grip change operation of engagement elements is performed.
Recently, automatic transmissions have been increasingly employing a multi-stage shifting mechanism so as to improve drivability and reduce the fuel cost by decreasing energy consumption. Such multi-stage shifting has been generally achieved by adding an acceleration or deceleration stage using an overdrive or under-drive gear to a shift mechanism consisting of a multi-staged planetary gear set. As another aspect, there has been disclosed in U.S. Pat. No. 5,106,352 a technology for achieving multi-stages by employing an input to a Ravigneaux type planetary gear set as two systems of high/low to achieve multi-stages.
In the multi-stage gear train as described above, the shifting speed conforming to the running state of a vehicle can be selected from a wider range of choices, thus causing the requirement for a complicated grip change of four elements as well as a simple grip change of two elements. As an example requiring such a grip change of four elements, there is shown a so-called skip shift from a number of shift stages to a particular shift stage at one time. In any case, when such a multiple grip change of four elements is performed, how to control the sequence or timing of the engagement and disengagement of each engagement element becomes a key issue. The failure of this control, if any, may cause the problem of degrading smooth advancement of shift operations within the transmission mechanism. In addition, if shift continuity is lost, there are problems of stepwise shocks during shifting, worsening of shocks at the end of shifting or an unnecessary increase of a shifting time.